Many vehicles are designed with a pair of levers to control vehicle speed and direction. Typically, one lever controls the speed of a left propulsion device, and another lever controls the speed of a right propulsion device. For example, the propulsion devices can be tracks or wheels. The two levers are often positioned close to each other so an operator can move them in unison with one hand and hold them stationary in a position corresponding to a desired speed and direction for the vehicle. Adjusting one lever differently than the other typically causes one side to move faster than the other, which leads to the vehicle turning, whether intentionally or not.
It is often necessary to drive such vehicles along a straight path, such as when digging a trench. Driving a straight path requires consistent input from the operator, who must monitor the direction of the vehicle and continuously make adjustments to the levers to keep the vehicle on course with minimal aberration from the desired straight path. Many factors out of the control of the operator can cause the vehicle to turn slightly, such as variations in terrain, changes in soil types, etc. This process tends to be fatiguing for the operator over long periods of time. The process can be especially fatiguing when the desired vehicle speed is less than the maximum speed of the vehicle, such that the operator has to keep both levers steady in an intermediate position along the available range of travel for the levers. It can be simpler to maintain a straight path at full speed, since an operator can push/pull both of the levers to the maximum limit of their travel and then hold them in that position with constant force applied against a stationary limiting surface. However, during working operations it is often impractical to operate a vehicle at full speed.